Increasing demand of electricity during peak hours and relatively high costs of retrofitting a power grid to the increased demand made the use of load leveling batteries in utility systems economically increasingly attractive. Among various types of known load leveling batteries, lead acid batteries have been relatively common. However, due to numerous problems associated with lead acid batteries (e.g., relatively low power density, environmentally problematic, relatively short service intervals, etc.), alternative load leveling battery systems have been developed.
For example, metal-sulfur batteries, and especially sodium-sulfur (e.g., KEPCO Kansai Electric Power Co., Inc. Osaka, Japan) and lithium-sulfur batteries have been employed as load leveling batteries. While metal-sulfur batteries generally exhibit significant increase in power density, all or almost all of them remain environmentally problematic. In another example, zinc-halogen, and especially zinc chloride and zinc bromine batteries (e.g., ZBB Energy Corp.) may be employed as load leveling batteries. The capacity of zinc halogen batteries is advantageously determined by the volume of the electrolytes, and therefore exhibits a favorable power-to-weight ratio in many configurations. Moreover, zinc halogen batteries tend to have a relatively long cycle time. Unfortunately, the use of halogens in such batteries is often problematic in various aspects. Among other things, zinc halogen batteries often require use of specialty materials (e.g., polymers in the electrode frames need to be resistant to corrosion by the halogens), thereby increasing the cost of production. Moreover, leakage of such batteries frequently poses a significant health and environmental hazard.
To overcome at least some of the problems associated with environmentally hazardous components, vanadium-based load leveling batteries may be employed, in which vanadium ions in the anolyte and catholyte cycle between 2+/3+ and 4+/5+ oxidation states. The use of vanadium significantly reduces potential threats to health and environment. Moreover, only the volumes of anolyte and catholyte generally limit the capacity of such batteries. However, using vanadium ions as redox couples typically limits the nominal cell voltage to 1.2V, thereby significantly increasing the size of load leveling batteries.
Thus, it would be desirable to have a load leveling battery that employs both a redox couple and battery chemistry with minimal health and environmental impact while providing an increased cell voltage. Among the most popular redox/electrical couples are those containing zinc. Zinc is regarded as the highest energy couple component that can be cycled in an aqueous room temperature battery and is therefore commonly used in numerous battery and power cell applications. Depending on the type of coupling partner such zinc containing batteries will exhibit various characteristic properties.
For example, zinc is coupled with carbon in most simple flashlight batteries to provide a relatively inexpensive and reliable power source. Although manufacture of Zn/C batteries is generally simple and poses only relatively little environmental impact, various disadvantages of Zn/C batteries and power cells exist. Among other things, the ratio of power to weight in commonly used Zn/C batteries is relatively poor, and such batteries are typically primary batteries. Consequently, Zn/C batteries are generally less desirable for load leveling applications. To improve the ratio of power to weight, alternative coupling partners and systems can be employ. For example, zinc can be coupled with silver to achieve an improved power to weight ratio. However, while silver as a coupling partner for zinc is environmentally substantially neutral and significantly improves the power to weight ratio, the use of silver is in many instances economically prohibitive and typically exhibits a poor cycle life.
In still further known batteries and power cells, zinc is coupled with still other metals such as nickel, copper, or manganese to provide a specific desired characteristic. However, and depending on the particular metal, new disadvantages may arise and particularly include environmental problems with manufacture and/or disposal, relatively low power to weight ratio, and undesirably low open circuit voltage.
Alternatively, oxygen may be employed as a gaseous coupling partner for zinc, thereby generally avoiding problems associated with toxicity, excessive cost for coupling partners, or spillage. Using air (i.e., oxygen) as coupling partner for zinc typically results in a relatively high power to weight ratio. Moreover, the zinc-oxygen system typically provides a relatively flat discharge curve. However, rechargeable zinc air batteries often exhibit relatively fast electrode deterioration. Moreover, experimental rechargeable zinc-air batteries have been built for use in automotive applications and typically use a liquid electrolyte that is recirculated via a pump. However, such systems are often impractical for load leveling applications.
An additional problem with zinc-air batteries often arises from the use of an alkaline electrolyte, which is typically disposed between a porous zinc anode and an air cathode formed of a carbon membrane. Unfortunately, the use of alkaline electrolytes in such electrodes frequently leads to absorption of carbon dioxide, and consequently formation of carbonates, which in turn tend to reduce conductivity and clog the pores in the active surfaces of the electrodes.
Thus, despite various advances in power cell technology, known systems and methods all suffer from significant problems. Therefore, there is still a need to provide compositions and methods for improved power cells.